The most commonly used procedure for the manufacture of continuous fiber product, such as strand, consists of drawing streams of attenuable material in attenuable condition, such as molten glass, from a bushing, the base of which is equipped with orificed tips. When cooling means are associated with the tips, stable separation of the streams is enhanced. The tips tend to minimize the phenomenon of "flooding" of the bushing base particularly at the time of a filament rupture during the fiberization operation. This flooding is manifested by the tendency of the molten material issuing from an orifice to spread over the surrounding undersurface of the base plate because of capillary action and the wetting effects of the plate. At the moment of filament rupture during the fiberization operation, the presence of tips tends to reduce the massive flooding of the base plate which would otherwise occur.
Taking into account the difficulties in manufacturing a bushing base with tips and also the significant space occupied by the tips, various efforts have been made to eliminate them and to replace the tipped bushing base with a generally flat perforated plate having a large number of orifices within a given surface area. The result of these efforts is the simultaneous attenuation of a larger number of filaments per bushing and consequently the possibility of the manufacture of new products, and a reduction in deformation problems of the bushing base which become progressively more significant as bushing surface area becomes larger.
Nevertheless, the advantage of an increase in the number of orifices per surface unit of the plate is, in practice, rapidly reduced by the flooding problem which occurs at the time of a filament rupture and which is produced much more easily since the distances between adjacent orifices are smaller, and the fiberization conditions, particularly the temperatures adjacent to the orifices of the plate, are less stable or less uniform.
In order to economically use bushings having a large number of orifices at a high density, and to achieve greater production capacity than that of the standard installations, it has been regarded as necessary to separate easily the glass into as many filaments as there are orifices, and to maintain this separation during the entire fiberization operation.
Various kinds of apparatus and processes have been proposed in efforts to obtain good fiberization stability with orificed plates, to decrease the number of filament ruptures and the risk of partial or total flooding of the plate, since flooding generally leads to lengthy delays in order to restart the fiberization operation. Among the various patents granted in this field, U.S. Pat. No. 3,905,790 is representative. This patent describes a flat-base bushing having a large number of orifices per unit of the surface and on the order of 30 orifices per cm.sup.2 or more. The formation of the filaments and maintenance of their separation requires a precise control of the temperature of the bushing base, by regulating the electrical energy provided and cooling it with a relatively large volume gas current continuously blown at high speed. The force of the gas blowing against the bushing base may cause breakage of fibers particularly when small diameter fibers are being drawn. Moreover, some turbulence of gas flow is inevitable and this turbulence leads to lack of uniformity in temperature on the base. This lack of uniformity introduces differences in viscosity of the glass being drawn from the orifices which leads to fiber breakage and differences in fiber diameter in fibers drawn from different orifices. Numerous modifications intended to solve the problems which have arisen when attempts are made using this technique have been proposed.
One modification described in French patent No. 2,297,194, consists of establishing and maintaining a significant temperature differential between the glass adjacent to the plate and the outer surface of the plate. The temperature of the plate is maintained at a temperature which is 28.degree. to 83.degree. C. lower than the temperature of the glass and this was found to reduce the tendency of the glass to flood. The temperature differential is obtained by a constant high volume and high velocity blowing of air on the bushing base.
Another modification, described in U.S. Pat. No. 4,033,742, consists of constantly blowing a high volume of high velocity cooling air from two series of delivery jets placed on both of the longer edges of the plate forming the bushing base. The delivery jets, which form an angle between 30.degree. and 60.degree. in relation to the plate, are placed at a distance from the center of the said plate which is between 5 and 127 cm, and release air jets at speeds of about 30 to 120 m/sec. The flow of air reaching the orifices is intended to be more uniform and an excessive consumption of air is intended to be avoided.
In spite of the modifications made, these attempts to eliminate tips suffer certain disadvantages which arise because of the use of blowing gas. In particular, it is difficult to obtain relatively uniform temperatures over the entire group of orifices and a constant and uniform cooling of the glass cones issuing from the orifices. In the case of flooding of the flat base, for instance after filament rupture, the restart of fiberization is still a long and complicated procedure. In effect it is necessary after complete flooding of the bottom of the bushing to increase the gaseous flow to obtain solidification of the glass, to thereafter proceed with alternate reductions and increases in the gaseous flow to gradually reduce flooding. All these operations readily result in thermal imbalance of the bushing, harmful to its proper operation. This type of process is all the more difficult if not impossible to implement when very small diameter fibers are intended to be manufactured since the force of the gas currents tends to fracture the tiny fibers. Furthermore, the fiberization apparatus is fairly complicated due to the presence of equipment for the supply of gas and of the system for regulating the various parameters relative to the gas blowing.
In an effort to decrease the tendency to flood the bushing, French patent No. 2,128,312 describes a process according to which a special gaseous mixture is directed toward the bushing base, whether it is made as a flat plate with orifices or as a plate having parallel, longitudinal channels perforated with a row of orifices. This process consists of continuously emitting an inert or nonoxidizing gas and a hydrocarbon gas which is decomposed by heat in order to deposit carbon and cause an adsorption of hydrogen on the bushing base. The carbon deposit, which results from the chemical decomposition of the gases in contact with the hot glass and hot bushing base, is supposed to decrease the wetting of the base by the glass, and tends to counteract the joining of the glass cones which are formed at each orifice. The complexity of the apparatus, which comprises perforated or porous tubular parts for the distribution of the gases and an entire supply system for the inert and hydrocarbon gases as well as the character of the gases utilized, prohibitively increase the cost of the fiberization installations.
Furthermore, the formation of glass fibers in a reducing environment in the presence of fine carbon particles can cause a degradation of the properties of the fibers.
Other proposals have been made, particularly in an effort to control the development of flooding after the rupture of a filament.
One such proposal is shown in U.S. Pat. No. 3,979,195 which connects each orifice to at least two adjacent orifices by fine grooves on the underside of the perforated plate. Hence, during the rupture of a filament, the glass is intended to flow in a groove to supply the filament drawn from one of the adjacent orifices. The filament, thickened by this application of glass, separates into two by the action of a jet of air permanently blown on the bushing base.
Besides the disadvantages of the permanent blowing of gas already mentioned, this system is delicate to implement. In effect, if the glass running through the channel is too cool, there is risk that when it contacts the adjacent cone it will cause a rupture of the adjacent filament, which will lead to an uncontrolled localized flooding; if the glass is too hot, the reduction in its viscosity increases the risk that it will migrate outside the channel and cause an equally uncontrollable flooding.
Another proposal is found in U.S. Pat. No. 3,982,915, which shows a modification of the structure shown in the patent identified just above. According to this patent, the orifices are perforated in a flat plate and arranged in zones containing several dozen orifices. At the heart of each zone the orifices are closely paired in groups of two or three, for example identified as a, b, and c in the patent.
When a filament descending from "a" is accidently broken, the glass flowing through "a" wets the metal separating the three orifices, preferably supplies the cones descending from "b" and "c", and is carried along by the latter two. With the permanent jet air action directed to the bushing base, a separation into three filaments re-occurs.
All the disadvantages inherent in the permanent blowing of a gas and in the very localized variations in the viscosity of the glass, possibly leading to a chain-reaction rupture or to an uncontrolled flooding are again present. In addition the start-up operation of the bushing initially requires the total flooding of the perforated base plate and it follows that the subdivision into filaments is long and delicate.
Another approach is disclosed in U.S. Pat. No. 3,574,581 which illustrates in FIGS. 8 and 9 a thin dimpled sheet 50 affixed to a base such as waffle plate 51. The dimples are said to be in the shape of sections of spheres and have one or a cluster of orifices (7 to 50) extended through each dimple. According to this patent glass under pressure in liquid form is extruded through the orifices preferably by means of a viscosity pump. Heating is provided by either heating of manifold 10, or the waffle plate 51. There is no joule effect heating of the dimpled sheet 50. Fibers are principally formed by extrusion and not by attenuation. The bushings are intended to be small as compared with prior art bushings (Col. 6, Line 37) and pressures of several hundred p.s.i. are contemplated.
Another more recent approach is shown in Belgian patent No. 876,072 of May 7, 1979. This patent discloses what amount to tips of cubical external configuration formed by machining grooves at right angles to each other into the base of the bushing. The patent states that it is essential that jets of air be blown against the bushing base.
From the foregoing it will be seen that, in the prior art, numerous techniques have been proposed for attempting to reduce the tendency towards flooding of the bushing, and thereby attempting to reduce the problems of establishing fiberization after such flooding.